U.S. Pat. No. 5,804,963 discloses an inductive position sensor including the above-described features. The position of the cursor relative to the scale along the path is derived from the measurement of the coupling between inducing and induced cursor conductors via the scale conductors. The magnetic fields generated and picked up by the cursor and scale conductors of such a sensor have a polarity reversal every pitch T along the measuring path. This sensor is insensitive to magnetic fields of homogeneous intensity and direction, such as generated by sources outside the sensor, the current they induce in two alternating hairpin turns being nil.
In a first embodiment disclosed in said patent, the cursor conductors are separated, strongly attenuating direct coupling between inducing and induced cursor conductors. But, as each conductor only faces a fraction of the scale surface facing the cursor, coupling between inducing and induced cursor conductors via the scale conductors is also reduced. Moreover, misalignment between scale and cursor gives rise to uneven coupling, altering measurements.
In a second embodiment disclosed in said patent, the cursor conductors are all interlaced: as each conductor faces the whole scale surface across the cursor, coupling between inducing and induced cursor conductors via the scale conductors reaches its maximum level. Misalignment between scale and cursor affects measurement only slightly, as it tends to affect all couplings the same way. But direct coupling between cursor conductors is important and leads to measurement errors getting larger with decreasing coupling via the scale, i.e. with increasing scale-to-cursor gap.
In order to increase measurement accuracy and to make it independent from said gap, direct coupling should be made negligible as compared to coupling via the scale, and the latter should remain unaffected by misalignment between cursor and scale.